[sazzy]

This the question:

Q) You need 100 ml of 200 mM phosphate buffer and you have two stock solutions from which you can make the buffer; 0.8 M NaH2PO4 (the acid form) and 0.8 M Na2HPO4 (the base form). You need to add 3 times more of the acid form than the base form to achieve the desired pH. How will you make this solution?

Here is my working, can someone tell me if this is correct and if not how to properly solve it, thanks::

want 200mM = 200mmole/L
                    = 200μmole/mL
                    = 20000μmole/100mL
                    = 20mmole/100mL

Need x3 more acid --> so need 15mmole acid, 5 mmole base.

Acid:
- Stock = 0.8M, we need 15mmole:
                                           0.8m = 0.8mol/L   = 15mmole/18.8mL


Base:
- Stock = 0.8M, need 5mmole:
                                        0.8M = 0.8mol/L       = 5 mmole/6.3mL